Radiation-sensitive compositions are routinely used in the preparation of imagable materials including lithographic printing plate precursors. Such compositions generally include a radiation-sensitive component, an initiator system, and a binder, each of which has been the focus of research to provide various improvements in physical properties, imaging performance, and image characteristics.
Recent developments in the field of printing plate precursors concern the use of radiation-sensitive compositions that can be imaged by means of lasers or laser diodes, and more particularly, that can be imaged and/or developed on-press. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available image-setters generally emit radiation having a wavelength of at least 700 nm, and thus the radiation-sensitive compositions are required to be sensitive in the near-infrared or infrared region of the electromagnetic spectrum. However, other useful radiation-sensitive compositions are designed for imaging with ultraviolet or visible radiation.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative-working printing plates, exposed regions in the radiation-sensitive compositions are hardened and unexposed regions are washed off during development. For positive-working printing plates, the exposed regions are dissolved in a developer and the unexposed regions become an image.
Usually lithographic printing plate precursors are supplied to customers in a stack of multiple elements, usually several hundred elements, with interleaf (or slip sheet) papers between adjacent precursors to prevent adhesion to one another and scratches on the imagable side. Without such interleaf papers, damage to the imagable side may occur during factory finishing operations, transportation, storage, or during use in plate setter devices.
There has been a desire to eliminate the use of interleaf paper to reduce waste and to simplify the loading process into imaging devices. One approach for doing this is described in EP 1,865,380 (Endo) in which silica-coated polymer particles are added to the topcoat. Organic filler particles are used in a similar manner in the materials of EP 1,839,853 (Yanaka et al.).
Copending and commonly assigned U.S. Ser. No. 12/336,635 (filed Dec. 17, 2008 by Ray, Mulligan, and Beckley) describes stacks of negative-working imagable elements that do not require interleaf papers because of the use of a thin topcoat layer over the imagable layer.
There remains a need to find a way to stack imagable elements such as lithographic printing plate precursors without the need for interleaf papers. While various particles have been incorporated into outermost layers of such elements when interleaf papers are omitted, there is a propensity for such particles to come out of the coatings, leaving voids in the outermost layer and settling in developer solutions used in processing the imaged elements.